The present invention relates generally to an indexing device which is mounted on a table or a pallet of a machine tool and the like and, more particularly, to an indexing device in which a worm gear mechanism is used.
One example of an indexing device of the above type is disclosed in U.S. Pat. No. 5,297,454 (corresponding to Japanese Patent Application Laid-Open Pub. No. 4-348,846 published Dec. 3, 1992). This indexing device accomplishes indexing by utilizing a known rigid tap function or synchronized tap function of the main drive spindle previously possessed by a machine tool (machining center). A clutch member mounted on the drive spindle of the machining center is engaged with a mating clutch member of an input shaft of the indexing device. First, with the synchronized tap function, the input shaft is rotated in its regular rotational direction as it is pushed in its axial direction. During regular rotation, a worm shaft is not rotated because of the functioning of a free-wheeling or one-way clutch provided between the input shaft and the worm shaft, whereby indexing is not carried out. When, after the drive spindle of the machining center stops, rotation in the reverse rotational direction is imparted to the drive spindle with the synchronized tap function, the worm shaft rotates, and an indexing table is driven in indexing rotation.
In this indexing device, after the worm shaft rotates, its rotation is stopped by a rotation correcting device. A positioning pin of a positioning shaft is mounted on the body structure of the indexing device in a slidable manner relative to meshing pawls of a clamp plate secured to the rear end of the table shaft of an indexing table. After the rotation has been stopped as above, the positioning pin is fitted in between the meshing pawls. In this manner positioning of the indexing table is accomplished, and at the same time any rotational moment applied to the indexing table during machining is countered.
Furthermore, in the rotation correcting device of the above described indexing device of the prior art, a driving gear is fixed to the lower end of the input shaft. This driving gear is meshed with a driven gear provided on the lower end of the worm shaft in a manner wherein the driven gear rotates unitarily with worm shaft but is slidable relative thereto in the axial direction. A triangular projection is provided on the upper end of this driven gear. A corresponding cut-out recess for engaging with and disengaging from this projection is formed in the body structure of the device. An axial shifting of the input shaft causes the driving gear to catch the driven gear and cause it to ascent along the worm shaft, thereby engaging and disengaging the projection and the cut-out recess. Thus the rotational advancing and retarding of the worm shaft due to the input shaft is corrected.
In the above described device, a positioning pin is fitted in between meshing pawls. In this mechanism, the positioning shaft is slidably fitted in the body structure. For this reason, abrasive wear occurs after a long period of service in the small sliding clearance between the body structure and the positioning shaft. Consequently, the positioning pin is apt to be inclined relative to the meshing pawls. This has given rise to the problem of a drop in the precision of positioning of the indexing table.
Furthermore, a single positioning pin fits in between the meshing pawls of the clamp plate. For this reason, when a finer indexing angle (for example, 1 degree) is sought, the sizes of the meshing pawls of the clamp plate and of the positioning pin become extremely small, whereby sufficient strength of these parts cannot be obtained, and their fabrication becomes very difficult if not impossible. Consequently there has been the problem of only indexing devices of relatively large indexing angles being available.
Further problems encountered in the case of the prior art indexing device are as follows. In the aforedescribed rotation correction device, the driven gear at the lower end of the worm shaft ascends and descends along the worm shaft and thus causes the projection to engage with and disengage from the cut-out recess in the body structure. For this reason, a large space for movement of the driven gear at the lower part of the worm gear becomes necessary. Consequently, the size of the entire indexing device in the worm shaft direction has tended to be large. Moreover, the input shaft together with the driving gear ascends and descends as it rotates with synchronized tap function, and the faces of the ends of the teeth of the driving gear cause the driven gear to ascend and descend in a state wherein they are in pressing contact with a plate provided on the driven gear. For this reason, the plate is easily worn by abrasion.